Paint-spray guns are most often supplied with paint from a container fixed under the spray gun; the paint is drawn into this container by the depression created at the outlet of the paint nozzle by the current of compressed air which pulverizes it and projects it onto the workpiece to be painted.
A lid is removably fixed to the container and said lid is itself fixed permanently to the spray gun. So as to allow the spray gun to be used for spraying paint downwards, for example onto workpieces placed on a table, or upwards, for example onto a "ceiling", the closure between the container and its lid must necessarily be airtight while allowing air to enter the container, as the paint is drawn therein and leaves therefrom. A vent hole is then always provided in the lid generally fairly close to the center thereof, on the handle side, so that spraying may be possible, either upwards or downwards; this arrangement allows the spray gun to be inclined more downwards than upwards, but it is more frequent to have to spray the paint downwards and this work requires the spray gun to be inclined. On the other hand, if the container is full and if the spray gun is very inclined, there is a danger of the paint flowing out through the vent hole.
If the paint flows through the vent hole when the spraying takes place from top to bottom, paint droplets may fall onto the workpiece being painted and cause, for this reason, inacceptable defects; the same is not true for upward painting.
To avoid this risk, attempts have been made to provide covers for paint-spray gun containers pierced with a vent hole which allows air to enter so as to compensate for the paint which is sprayed, but does not let the paint flow through if it is submerged following too great an inclination of the spray gun.
Numerous solutions have been proposed with a view to obtaining this result; they generally give satisfaction insofar as the vent hole provided for the air intake has not been stopped up by the paint, which causes interruption of the paint supply at the spray gun; however, in all the solutions provided up to present, cleaning of the stopped-up air intake is difficult, if not practically impossible, because it is a question of very small section passages, often of a great length, and curved to a greater or lesser degree.